The prior art encompasses various sorts of flowmeters which are built on different physical principles. Fluid flows may, for instance, be determined by measuring the pressure drop with the aid of an obstruction meter, measuring the vertical position of a suspended body with the aid of a rotameter, or measuring the transit time of an ultrasonic pulse through the fluid flow with the aid of an ultrasonic meter.
As a rule, a flowmeter contains a transducer emitting a measurement signal which essentially, in view of the method of measurement employed, represents the instantaneous flow rate affecting the transducer, as well as electronics for filtering the measurement signal and converting it to a flow output signal, which is adapted to a receiving unit, for instance a control system, and which often consists of an average value of the instantaneous flow rate for a certain period of time.
A common feature of most flowmeters is their sensitivity to the character of the fluid flow whose flow rate is to be measured, such that the output signal from the flowmeter is altered if the character of the fluid flow is altered, in spite of the fact that the flow rate remains the same. This phenomenon is often labelled "installation effects".
One distinguishes between static and dynamic installation effects. Thus, static installation effects are due to time-independent alterations of the flow profile, for instance caused by T-pipes, step-like diameter changes, tube bends or valves.
Dynamic installation effects, on the other hand, are due to time-dependent alterations of the fluid flow whose flow rate is to be measured. These time-dependent alterations, which for instance may consist of flow pulsations, are created by active components, such as pumps, compressors and rapid control valves. If, in a pulsating flow, the flowmeter averages the instantaneous flow rate during a period of time much exceeding the pulsation period, incorrect measurement results may be obtained.
Flowmeters are calibrated for a given flow profile, namely the fully developed flow profile obtained when an incompressible fluid has flown through a straight, circular conduit of a certain length. However, a real-life installation often differs from the calibration installation. As a result, the flowmeter may measure a flow having a flow profile other than the one for which it is intended or calibrated, which thus may result in an incorrect output signal from the flowmeter.
Furthermore, the flowmeter is calibrated for a certain flow-rate range, and it goes without saying that the flowmeter may emit an incorrect output signal if the measured flow rate is outside the calibrated range.
Moreover, certain types of flowmeters normally sample at a fixed sampling frequency. If the fluid flow pulsates, an unsuitable sampling frequency may lead to aliasing effects, such that an incorrect output signal is obtained.
Unless it is detected that the flowmeter measures incorrectly, the incorrect flow-rate values may lead to, for example, incorrect debiting in district heating systems, incorrect regulations in the process industry or incorrect dosages when mixing medicine or food. It is thus of the utmost importance that a flowmeter measures correctly or that one at the very least is able to find out whether it does so or not.
The installation effect problem being well known since long back, considerable efforts have been made to solve this problem.
One way of solving this problem, or at least reducing it, is to condition the flow such that a fully developed flow profile is obtained at the flowmeter. This may, for instance, be done by arranging a straight tube of constant diameter in front of the flowmeter, which however is not always possible from considerations of space, or by arranging a so-called "flow straightener" in front of the flowmeter. However, such a device introduces a pressure drop into the system, which is not always acceptable. If there are pulsations in the flow, these may further be reduced by the introduction of a damper.
Another way of solving the installation effect problem is by calibrating the flowmeter in situ or simulating the existing flow conditions. Should, however, the flow conditions be altered after the calibration, for instance as a result of changes in the system or changes in the function of a component, there is nevertheless a risk that the flowmeter will yield incorrect measurement values. In addition, it is not always possible to perform in-situ calibration, and this is at all events a time-consuming and expensive operation.